Unukalhai tends to be stoic and businesslike at all times, revealing little of his personality and motives. Only once, when questioned of his true goal by Krile, does he show a flicker of emotion, which he quickly quells by the next time he is encountered. He seems to hold disdain for the Garlean Empire and their reckless attempts to control the power of the Primals. He often speaks of his master, on whose behalf he claims to have arrived, but does not reveal who or where this master is.

Unukalhai first appears after the defeat of Thordan VII, in the quest "Gods of Eld". The Adventurer is told that a visitor is waiting for them in the Scions' headquarters, The Rising Stones, and arrives to find a young, masked man waiting for him. Introducing himself as Unukalhai, he claims to have been sent by his master to aid the Scions in facing the coming threat of the "Eikons", Primals that were summoned during the Third Astral Era. Known as the Warring Triad, they were defeated and imprisoned by the Allagan Empire, and sealed on the floating island of Azys Lla. The actions of Archbishop Thordan VII, whom tried to gain their power for himself, has weakened their seals, making it possible for them to break free - a process the Garlean Empire may hasten in their attempts to uncover the secret of binding primals. This explanation is corroborated by Urianger, who arrives to confirm Unukalhai's story and vouch for his character, saying that he can be trusted for now.

Before the Triad can be dealt with, the Adventurer is told that they must first defeat the primals Bismarck and Ravana, who have both been recently resummoned by their respective worshipers. Unukalhai states his intention to assist with this, offering instructions on how to confront each of them, including two enigmatic gems associated with each primal.

When it becomes time to deal with the Warring Triad directly, Unukalhai instructs the Adventurer to journey to Azys Lla alongside Krile, Urianger and Y'shtola. There, he leads them to Containment Bay S1T7 where Sephirot, the first of the Triad to risk being awakened, is held. While Y'shtola and Urianger distract the Garlean forces patrolling the area, the adventurer, Unukalhai, and Krile unseal and defeat Sephirot within its containment.

While leaving the area, Unukalhai is approached by Regula van Hydrus. Unukalhai criticizes the Garlean's attempts to control the power of the primals. Regula merely rebuffs this lecture, but leaves without a fight. Soon after, Krile attempts to question Unukalhai's motives and background, and though he shows a brief flicker of emotion, he manages to quell it without revealing anything of his past.

After returning to The Rising Stones, Unukalhai reveals more of the background of Sephirot, and offers the Adventurer an Allagan Tomestone that will allow them to enter a combat simulation pitting them against a stronger version of the Eikon.

Unukalhai brings word to the Adventurer that Regula van Hydrus has managed to break the wards that Urianger used to seal off the Aetherochemical Research Facility. Conferring with the Scions, Unukalhai states his belief that van Hydrus is after the "Genesis Expression", one of the most powerful pieces of Allagan technology, and recommends pursuing and, if possible, slaying him. The Adventurer, Unukalhai, Urianger, Y'shtola, and Krile journey inside the facility, fighting off waves of Garlean troops. Along the way, Krile is injured, and Unukalhai stays behind to protect her, earning her trust and gratitude. Although the Adventurer is successful in defeating van Hydrus in battle and preventing him from retrieving the Allagan tomelith, he is able to escape before being slain.

After the mission is complete, Urianger asks to speak to the Adventurer and Unukalhai in private, revealing that he deduced that it was Unukalhai himself that broke the seal on the facility, hoping to lure Regula van Hydrus into a trap where he could be killed. After Unukalhai admits this to be the case, Urianger chides him, stating that had he trusted the Scions enough to tell them his plan, it may have succeeded. Despite this, Urianger asks the Adventurer to forgive Unukalhai for this deception, and to keep it a secret from the other Scions.

In order to atone for his lack of honesty, Unukalhai decides to share some of his own personal history with the Adventurer, and reveals that he, too, possesses a form of the Echo . He states that he originally came from a conflict ridden land, where he initially learned magic in order to survive, until his master found and recruited him. He then tells the story of another world, where people had believed they could harness the power of Primals using Auracite. However, this eventually led to the destruction of their world. He explains that it is the fear that something like that could happen to Eorzea that causes him to despise van Hydrus and the Garlean Empire.

Unukalhai informs the Adventurer that the second of the Warring Triad, the goddess known as Sophia, is about to awaken. Upon again traveling to Azys Lla with Urianger, Y'shtola and Krille, they are surprised to find that several of Sophia's thralls, who had been sealed along side her by the Allagans, have awoken. While Urianger and Y'shtola deal with the thralls, Unukalhai, Krille and the Adventurer enter Containment Bay P1T6, where Sophia in contained, and defeat her in combat.

After the conclusion of the battle, while the Scions prepare to leave, Unukulhai stays behind momentarily in order to shut off the life support for Sophia's remaining thralls, killing them, in order to prevent the possibility of another summoning. When later asked about his actions by Y'shtola, he explains that he knew it had to be done, but did not want the Warrior of Light to have to sully themselves with such a distasteful task. Y'shtola explains that all of the Scions, including the Adventurer, know what has to be done in the battle against the Primals, and are willing to do what is necessary, including the slaying of thralls. Unukulhai admits that he had been mentally placing the Adventurer on a pedestal, seeing them more as the embodiment of an ideal than as an individual, and promises not to underestimate their resolve in the future. He then presents the Adventurer with a Tomestone containing data on Sophia, allowing them to challenge a simulation of a stronger version of the goddess.

Unukalhai is the traditional name of Alpha Serpentis, the brightest star in Serpens, a constellation often represented as being held by Ophiuchus. As Ophiuchus is the symbol associated with Elidibus, this very likely hints to a connection to the Final Fantasy XIV character of the same name.